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I received an email yesterday, and I'll paraphrase the question:
"When I salvage 18 rares I seem to make a 40 silver to 1 gold profit, but when I try to salvage 100 of them the next day I am losing money. Why?"
There are numerous reasons for why this is failing, and some of them might surprise you.
Scenario #1: Simple Mathematics
Random is random, but when you play a small number of times then it is possible to see incredible success. This success is much higher than the average amount of profit (or loss) you'd make playing the salvage game long term. So if you are meant to get around 1 gold per 100 tries and you only make 10 attempts, you could get lucky and make like 2 gold. But if you were to go another 90 times you should end up making around that 1 gold. Truly profitable strategies make you gold over the long haul, not just randomly if you're lucky.
Next time you go to the casino, quit the moment you're ahead. You want to do this because often times you have a pretty decent chance to win, but it's lower than 50%. So you'll win after a few tries and end up with more money than you started, but if you continue playing than mathematically you will slowly lose everything. Same goes for salvaging these rares.
Scenario #2: Salvage Trends
People salvage for the achievement, but they also salvage for profit. Whenever the salvaged goods become profitable then people will buy up all the cheap rares until the price matches the salvaged goods price. You did not mention the actual cost of the rares. Were they higher on day 2 compared to day 1? Also, were the salvaged items less profitable on day 2? Odds are that is exactly what happened.
Scenario #3: Online Speculation
A lot of websites, especially this one, cause changes in the marketplace by mentioning items, events, patches, etc. Sometimes you will stumble upon the effects of one of these sites. If you do notice something odd, google the item for the past 48 hours and see if something pops up! Use keywords like speculation, gw2, trading post, etc.
How do you prevent getting screwed by the randomness of salvaging? Do the math on your odds of success and pay attention to not just the price of the rares but also the items they turn into! I use crude salvage kits as well to save on the cost (bought with karma).